Enter the days you will genuinely rely on this data, which is usually your travel days. If the plan runs ten days and you are away seven, dividing by ten produces an artificially tight budget and needless rationing. If you do intend to keep using it in a second country for the full ten days, then ten is right.

Yes, and you should. Read the real usage for the eSIM line in your phone's statistics, then run the remaining data against the remaining days. The new figure reflects where you actually are, which beats clinging to a budget set before you left home.

No. It is purely your own planning aid: nothing is sent to a supplier and no limit is applied to the plan. Going over on a given day triggers nothing from the plan's side. The only consequence is less data for the days that follow.

They measure at different points: the phone counts what the operating system sees, the network counts what crosses its own equipment. A small gap is normal and does not mean either is faulty. Plan with a margin rather than to the last megabyte.

Budget them separately: each plan has its own allowance and its own expiry, and data does not flow between them. Decide which days belong to which plan first, then work out a daily figure for each window. If you are unsure which to use first, let the expiry dates decide.